Regional and stress drop effects on aftershock productivity of large megathrust earthquakes

The total number of aftershocks increases with main shock magnitude, resulting in an overall well‐defined relationship. Observed variations from this trend prompt questions regarding influences of regional environment and individual main shock rupture characteristics. We investigate how aftershock productivity varies regionally and with main shock source parameters for large (Mw ≥ 7.0) circum‐Pacific megathrust earthquakes within the past 25 years, drawing on extant finite‐fault rupture models. Aftershock productivity is found to be higher for subduction zones of the western circum‐Pacific than for subduction zones in the eastern circum‐Pacific. This appears to be a manifestation of differences in faulting susceptibility between island arcs and continental arcs. Surprisingly, events with relatively large static stress drop tend to produce fewer aftershocks than comparable magnitude events with lower stress drop; however, for events with similar coseismic rupture area, aftershock productivity increases with stress drop and radiated energy, indicating a significant impact of source rupture process on productivity. Plain Language Summary Earthquakes produce aftershocks, but how many aftershocks does any one earthquake produce? In general, higher magnitude earthquakes produce more aftershocks, but there is considerable variation around this general behavior. Here we show that some of the variability is due to regional differences. Large, Western Pacific earthquakes produce more aftershocks than analogous events in the Eastern Pacific. An additional factor is the stress released in the earthquake. For earthquakes with similar size ruptures, the ones with greater stress released produce more aftershocks. The results have direct implications for aftershock forecasting and more generally help zero in on the processes of aftershock generation. Key Points Higher stress‐drop megathrust earthquakes produce fewer aftershocks than lower stress‐drop events of similar magnitude Productivity increases with stress drop and moment‐scaled radiated energy for events with similar main shock rupture area Large megathrust earthquakes located in western Pacific subduction zones have higher aftershock productivity than in eastern Pacific zones